This invention relates to a fuel control apparatus for a fuel injection system of an internal combustion engine which measures the rate of air intake into the engine using an air flow sensor and controls the supply of fuel to the engine based on the output of the sensor.
In an internal combustion engine which employs a fuel injection system, it is conventional to dispose an air flow sensor (hereinunder abbreviated as AFS) upstream of the throttle valve of the engine and to calculate the rate of air intake per each engine revolution based on the output of the AFS. The injection of fuel is then controlled based on the calculated intake air flow rate.
Since the AFS is disposed upstream of the throttle valve, the air flow rate measured by the AFS does not always coincide with the actual air flow rate into the engine cylinders. In particular, when the throttle valve is abruptly opened, there is a sudden increase in the air flow through the AFS, but due to the provision of a surge tank between the throttle valve and the engine cylinders, the increase in the air flow rate into the cylinders is more gradual and of a smaller magnitude than that into the AFS. Accordingly, the air flow measured by the AFS is greater than the actual air flow into the engine, and if the fuel supply were controlled based solely on the value measured by the AFS during a single brief period when the air flow rate was in transition, the fuel-air mixture would be overly rich. Therefore, the actual air flow rate into the engine cylinders is calculated as a weighted average of the value measured by the AFS over several periods, such as during two consecutive half-revolutions of the engine, and more accurate fuel control can be performed.
However, when the AFS is of the Karman vortex type, it produces output pulses whose frequency varies with the intake air flow rate, which depends upon the load of the engine. The frequency of the output typically varies from 40 to 1200 Hz. Furthermore, the frequency of the AFS output greatly fluctuates under a heavy load. At such a high frequency, a computer for processing the output signals from the AFS can not keep up with the output signals, the amount of intake air per engine revolution can not be accurately detected, and the fuel supply can not be correctly controlled.